If taken literally, a simple comparison can be made. In 2012, the US murder rate was 4.7 per 100,000, a total of14,827. Arrests amounted to only 7,133. Using only people who were arrested (not just convicted) would lower the US murder rate to 2.26 per 100,000. If about 2/3rds of those arrested are convicted, that would lower the rate to about 1.5 per 100,000. In 2011, the England and Wales had a homicide rate of about 1.15 per 100,000 (see also here). (Note there have recently been some serious allegations that British police have been ordered to hide reported crimes, though I am not sure how they could bias the homicide rate.)

Yet, I see other discussions that are somewhat less clear. Here is another report from the House of Commons that was released in 1999 (it also contains data on how much this adjustment changes the counted number of homicides in England and Wales).